Orthogonal frequency division multiplexing (OFDM) is a communications technique that divides a communication channel into a number of equally spaced frequency bands. A subcarrier carrying a portion of the user information is transmitted in each band. Each subcarrier is orthogonal with every other subcarrier, differentiating OFDM from frequency division multiplexing (FDM).
As a digital transmission technique, OFDM uses a large number of subcarriers spaced apart at slightly different orthogonal frequencies. Each subcarrier can transmit a lower-speed signal, all of which are aggregated at the receiving side into the original high-speed signal. Lower speed signals are more easily deciphered at the receiving end.
In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the multi-path interference among the channels and symbols comprising the date stream. Less importance is placed on perfecting individual channels.
A problem with OFDM is that non-linearity in the signal chain will cause intermodulation distortion that raises the noise floor, may cause inter-symbol interference, and generates out-of-band spurious radiation. The linearity requirement is demanding, especially for transmitter RF output circuitry where amplifiers are often designed to be non-linear in order to minimize power consumption.
Non-linearity is also a problem for FDM where in many practical communication systems, the power amplifier in the transmitter has nonlinear characteristics, and nonlinear amplification leads to the creation of out-of-band spectral components and harmonics that interfere with other FDM channels. Thus, it is necessary to use more complex linear amplifiers in FDM systems.
So far the discussion has been about non-linearity in the equipment used to transmit and receive a signal. The case where the transmission medium itself is non-linear is even more challenging. In this case even if the equipment is perfect the transmission medium itself will cause signal mixing and harmonic distortion to occur. This is a serious problem to be overcome if multiple channels are required or desired in such a system.
In the prior art of which applicants are aware, U.S. Pat. No. 6,760,300 to Eberle et al. discloses a wired or wireless communication system and methods based on OFDM modulation technique, which contemplates non-orthogonal frequency division multiplexing as a training signal for only the first part of the signal, and using OFDM for the second part of the signal. Eberle et al. does not contemplate the ability to send signals on a non-orthogonal frequency “cluster” which fills in the spaces between an adjacent pair of, or otherwise includes at least one of the non-orthogonal base or anchor frequencies, nor does Eberle teach the avoidance of intermodulation distortion as taught herein.